The present invention relates to a magnetron which oscillates microwaves, and more particularly, to a magnetron which can effectively reduce spurious mode oscillation.
In recent years, spurious radiation control in microwave radiation devices has been getting severer. In this trend, it is hard for a magnetron which is oscillating tube to reduce spurious radiations because of its oscillating mechanism. In spite of this disadvantage, magnetrons are widely used in transmission devices such as radars because of their being inexpensive, easy to handle and having a large output. Therefore, magnetrons with reduced spurious radiations are under development.
In general, magnetrons oscillate in the main oscillation mode referred as a π mode in which adjacent hollows have a confined phase difference of π radian. However, magnetrons have a lot of resonant modes other than the π mode because of their structure. The largest spurious component radiated by vane and strap magnetrons is the component referred as the (π−1) mode which produces unnecessary radiations of a frequency of about 1.1 to 2 times the oscillation frequency of the π mode. The output level ratio between the π mode and the (π−1) mode is usually −30 to −50 dBc or so (the decibel value compared with the fundamental wave level); however, in order to avoid interference or noise source, it is necessary to reduce spurious radiations at a higher level.
In order to reduce these spurious radiations, there is a circuit configuration as shown in FIG. 5 where the spurious oscillation waves transmitted from the antenna 51 of the magnetron 50 into the waveguide 52 together with the fundamental wave of the π mode is attenuated by the filter 53 connected to the waveguide 52. In this illustration, the reference numerals 55, 56, and 57 represent anode shell, vane, and strap, respectively.
The available filters include a band-pass filter, a low-pass filter, and a band-rejection filter through which pass the frequency of the main oscillation mode, but do not pass the spurious component, the π−1 mode, (See Japanese Unexamined Patent Publication No. 2001-35399, for example). When these filters are placed, the fundamental wave of the π mode which has the passband width can pass without problems, but the spurious oscillation waves of the π−1 mode which is higher in frequency than the π mode are rejected by the filters without being transmitted in the direction of the transmission antenna.
As described hereinbefore, a magnetron requires that filters be placed in the radar set because spurious oscillation waves are radiated from the output. However, the radar set, which is usually installed at a high position in a ship or the like, must be designed to be lightweight and small. The placement of filters do not comply with the requirement of the magnetron. In addition, the filters must be processed in high precision in order to secure the amount of attenuation of waves other than the fundamental wave and to pass the fundamental wave without being attenuated. This causes the problem of making magnetrons costly.
The present invention, which has been contrived to solve these problems, has the object of providing a magnetron which can reduce spurious oscillation such as the π−1 mode which presents a problem particularly in magnetrons and can reduce spurious radiations without placing filters.